The Volcanic Ash Forecast Transport and Dispersion (VAFTAD) model was fully transferred to
the NWS/NCEP Cray computer system on 22 April 1999 at 1200 UTC. For approximately the
last 7 years the VAFTAD model has run operationally on a NOAA Air Resources Laboratory
(ARL) computer which was not in a suitable environment for use in an operational 7/24 program.
Also accomplished during the transfer was the introduction of a new "reduced ash" version (Figs. 1 and 2) in addition to the standard
ash version (Figs. 3 and 4) ) of the VAFTAD model. The standard ash
version was reviewed several years ago by Nick Heffter (ARL). In 1988 NOAA and the FAA
signed a MOU for Volcanic Hazards Alert program. This resulted in a trajectory model which
was run on the then NMC 9000 computers to support jet aviation interests. Information was
transferred to the airlines via phone fax. The trajectory model was replaced by the development
of the VAFTAD model on the ARL computer.

1. THE VAFTAD MODEL - BRIEF DESCRIPTION

The VAFTAD model is a time dependent 3-dimensional volcanic ash transport and dispersion
model. It was developed for global forecasting of volcanic ash in response to volcanic eruptions
around the world. The "visual ash cloud" is depicted on the model output charts since ash
density and particle size hazardous for aviation operations have not been determined. The focus
of the VAFTAD model is for the in-flight hazard to aviation operations in support of the NOAA
Volcanic Ash Hazards Alert Program.

particle fall added to the vertical advection each 1 hr time step. Ash particles fall
according to Stoke's Law and include a slip correction.

Visual ash cloud depiction

calculated concentrations have been correlated with satellite imagery for defining the
visual ash cloud

model computation includes the magnitude of the volcanic eruption as determined from
the eruption ash cloud height

VAFTAD products

The VAFTAD model charts consist of a series of maps showing the instantaneous ash cloud in
time and space starting 12 hours after eruption time and continuing in 12 hour intervals through
hour 36 or 48. The 4 panels in any column are for a single valid time after the eruption time.
Individual panels are for layers applicable to aviation operations and are identified at the side
panel with upper and lower flight levels (FL) in hundreds of feet. The bottom panel is a
composite from the SURFACE to FL550. For each column, the forecast valid time separates the
upper three panels from the composite panel. Volcano eruption information is at the lower left.
A description of the input meteorology is at the lower right. The visual ash cloud symbol and run
description are at the lower center. The volcano site is depicted on the maps by a circle
containing cross-hairs.

Ash cloud reduction

Included in this implementation is the relatively new (Dec. 1998) ash reduction alternative to the
standard VAFTAD ash output, which was modeled on larger type eruptions . Most (90%) of the
volcanic eruptions handled by the Washington and Alaskan Volcanic Ash Advisory Centers
(VAACs) are of the short duration (visible on satellite imagery less than 18 hours) and low mass
variety, or "puffs". The standard VAFTAD model simulation often results in cloud excessive ash
concentrations (CEAC), which makes it difficult for the model to handle these "puffs" with
accuracy, especially when there are large, but unknown, concentrations of steam in the eruption
column. Small (1), medium (2) or large (3) ash reductions can be manually entered in the model
inputs to more closely simulate the actual ash cloud observed in satellite imagery. Differences
among the standard and the various ash reduction runs reflect some of the uncertainty in defining
the eruption column. The result of using the reduction mode is usually a more accurate
VAFTAD forecast.

The ARL is responsible for continued verification, research and development for modeling
volcanic ash clouds and making model changes. ARL also assists implementing VAFTAD
upgrades at NCEP.

NCEP is responsible for maintaining the VAFTAD model in a 7/24 operational environment on
its computer system. NCEP also runs the AVN global model four times a day in 6 hour cycles
for input to the VAFTAD model while also providing other meteorological models for further
VAFTAD model development. AVN analysis fields for the last 48 hours are also run and stored
every 6 hours for VAFTAD use. The AVN and VAFTAD models reside in normal production
suites where both production and communications are fully supported by a staff of programmers
and technicians.

VAFTAD suite

VAFTAD analysis job (meteorology pre-processor, run 4 times a day)

VAFTAD forecast job

VAFTAD fax job

The NCEP Senior Duty Meteorologist (SDM) or the forecaster at the Anchorage Aviation
Weather Unit (AAWU) can set up a file containing the inputs for the VAFTAD model. The
SDM then runs the model by requesting the computer operator to release the VAFTAD. Upon
model completion a postscript file is created and the output is reviewed for consistency with the
meteorology fields, satellite imagery, and any observed data in the vicinity of the ash cloud. The
postscript file is also automatically sent to AAWU and the NESDIS Satellite Analysis Branch. A
standard VAFTAD or an ash reduction VAFTAD may be run before the SDM has the operator
release the VAFTAD fax job which communicates the product to the customers. The model may
be rerun later if additional more accurate information is received. If the model is not verifying
properly or if the visible ash cloud no longer exists, an Alert Ended VAFTAD is also run and
issued. The SDM makes recommendations to ARL regarding model improvements.

3. VAFTAD INTERNAL USERS AND CUSTOMERS

It could be said that the Washington VAAC and Anchorage VAAC are the primary users of the
VAFTAD model, and having the VAFTAD model run on the NCEP operational computer
system adds to the convenience and stability of VAAC operations. In 1997 the International
Civil Aviation Organization (ICAO) mandated graphical support for volcanic ash guidance,
forecasts and warnings for aviation from many of the national meteorological services across the
world. The Washington and Alaskan VAACs were officially established by the end of that year.
The Washington VAAC is responsible for providing volcanic ash guidance for all the US flight
information regions (FIRs), excluding the Alaskan FIR, southward to 10 degrees south of the
equator. It also tries to support the Buenos Aires VAAC, which is still not operational, over
South America. For the two years prior to 1998, the Washington VAAC was tasked with
covering the world and averaged almost 350 VAFTADs per year. With the inauguration of the
other VAACs around the world, and "WATCH" VAFTAD runs being made available on the
Internet instead of being issued by the VAAC, the number of VAFTADs issued has dropped in
1998 to just over 150. The NESDIS Satellite Analysis Branch (SAB), part of the Washington
VAAC, issued over 1000 Volcanic Ash Advisory Statements (VAASs) during 1998 in text
format. The VAFTAD helps NESDIS/SAB determine where ash exists (analysis), while the
NCEP/SDM determines where ash is going (forecast).

Other VAAC VAFTAD customers

18 hour graphical guidance for met watch office SIGMET preparation

guidance and outlooks to 48 hours for the FAA.

guidance and outlooks to 48 hours for the airlines

military and other government agencies

backup for other VAACs

other NWS forecast offices (currently not part of this program)

4. CUSTOMER NEEDS - HOW THEY ARE SATISFIED BY THE IMPLEMENTATION

Volcanic ash presents a significant hazard to jet aircraft operations. Modern day eruptions have
nearly downed two Boeing 747s in incidents near Alaska and Indonesia. About two years ago
during a satellite eclipse an Air Canada DC-10 had a problematic encounter from a new eruption
on Montserrat in the Caribbean. Volcanic ash clogs jet engine fuel injectors and Pitot tubes,
while scouring windscreens until they lose there translucency. Often present sulphur dioxide gas
is corrosive to aircraft materials. The aviation community is desperate for any ash guidance they
can receive in a timely fashion. At risk are the lives of passengers and crew and the loss of
millions of dollars in maintenance costs to just a single aircraft. The Washington VAAC issued
over 150 VAFTADs and over 1000 VAAS text messages during 1998 in support of both US and
foreign met watch offices, flight services and airlines.

The VAFTAD is officially released when the SDM runs the VAFTAD fax job out of the
VAFTAD job suite. The VAFTAD graphics are communicated as a 6 bit fax graphic file to
National Weather Service Telecommunications Gateway for official dissemination as is other
NCEP fax production. Gateway converts the fax file for WAFS and the NWS web server.